Preheat the oven to 425°F / 220°C.
In a small saucepan, combine the sugar and water.
Heat over medium heat, swirling the pan (do not stir) until the sugar dissolves completely.
Continue to cook, swirling occasionally, until the caramel turns a deep nut-brown color.
Immediately remove the saucepan from the heat and carefully swirl in the sherry vinegar until it's fully combined with the caramel.
Working quickly, pour the caramel evenly into the bottom of four 4-ounce ramekins.
Let it cool for about a minute.
Assemble the tarts by placing 1 roasted garlic clove, 3 pitted olives, and one roasted tomato half (cut-side down) into each ramekin on top of the caramel.
Season with a pinch of salt and pepper.
Spoon the onion confit evenly over the tomatoes in each ramekin.
On a lightly floured surface, roll out the puff pastry.
Cut four rounds that are slightly larger than the diameter of your ramekins.
Place one pastry round over each ramekin, gently tucking the edges down inside the rim.
Place the ramekins on a baking sheet and bake for about 20 minutes, or until the pastry is puffed, golden brown, and cooked through.
Let the tarts cool in their ramekins for 1 to 2 minutes. To serve, carefully run a knife around the edge of the pastry and invert each tart onto a plate.
Serve warm or at room temperature.
